1. Field of the Invention
The present invention relates to a device which is capable of automatically detecting a downed conductor in a three-phase four-wire multi-grounded distribution system and which acts to protect the line from arcing ground faults and overcurrents.
2. Prior Art
Fuses, breakers, sectionalizers, and automatic reclosers are conventional protectors of this kind which operate when the distribution lines allow overcurrents. These protectors can detect overcurrents when the overcurrents exceed the operating currents of the protectors. However, the conventional protectors cannot always protect the downed conductor in the three-phase four-wire multi-grounded distribution system.
The present applicant has already disclosed a new branch device in Japanese Patent Publication No. 38089/1982 in use for a three-phase three-wire ungrounded distribution system. This branch device comprises a no-voltage detecting circuit to lock in the branch device which has a breaking capacity less than the maximum prospective short-circuit current of the distribution line and to open the branch device under no-voltage conditions of the distribution line, an overcurrent detecting circuit for detecting an overcurrent which exceeds the breaking capacity of the device, a first activating circuit for activating a trip mechanism, a second activating circuit which is charged by operation of the overcurrent detecting circuit and has a given power circuit to the tripping mechanism, and a relay. When the no-voltage detecting circuit operates, the relay is energized by the second activating circuit and operated for a given time if the overcurrent detecting circuit is operated. When the overcurrent detecting circuit does not operate, the trip mechanism is activated by the operating relay to open the branch device. In this case, operation of the no-voltage detecting circuit is not involved to any operation of the branch device.
When this branch device locks in, if a large current flows, the device is opened after associated source side protective device opens and under no-voltage condition of the line. If no large current flows, i.e., the current is less than the predetermined operating current of the overcurrent detecting circuit, then the branch device opens immediately to isolate the branch circuit. Therefore, the electric power is continuously supplied to all healthy sections without any interference. The power failure section is limited to only the faulty section. Hence, it can increase the reliability of the power supply. In addition, it is easy to discover a faulty point. Moreover, the branch device is not required to interrupt abnormal high short-circuit current and is capable of using an inexpensive device which does not need high-breaking capacity of the maximum prospective short-circuit current of the distribution line.
An arcing ground fault current which is one of abnormal currents has a unique ground fault phenomenon. In particular, when a distribution line breaks and falls down to the ground, an arc jumps to the ground but the fault currents are restricted. These faults are described as high impedance faults at the point of the fault. This impedance is often sufficient to restrict current flow values which are low fault levels. From the above example, the ordinary overcurrent protection devices do not protect such high impedance faults. Therefore, some faults will remain undetected and the power supply is resumed. If such an undesirable phenomenon is not remedied, the downed conductors could prove fatal if they come into contact with a human being. Further, fire or other serious accidents may occur.
When the above-described arcing ground fault occurs, it is common practice to disregard the abnormal current, permitting the persistence of supply of electric power, because the current is less than the operating current of the protective devices. In a three-phase three-wire ungrounded distribution system, the abnormal current can be detected by the method described in the above-cited Japanese Patent Publication No. 38089/1982.
In the U.S.A., R.O.C., South Korea and other regions, three-phase four-wire multi-grounded distribution systems are adopted, since no abnormal voltage is produced when a ground fault occurs, permitting the insulation level of a transformer or other device to be made lower than the insulation level used when a three-phase three-wire ungrounded system is employed. In three-phase three-wire ungrounded systems which are widely adopted in Japan for lines through which voltages less than 30 kV, the magnitude of ground fault currents are relatively low value but the ground fault currents can be detected and can be distinguished from load currents. The ground fault current can be detected properly only by a core-balanced current transformer which can sense the ground fault current. On the other hand, in the three-phase four-wire multi-grounded distribution system, it is impossible to connect a core-balanced current transformer to a tap point of the distribution line to sense the ground fault current since there is only one power distribution line, and therefore the core-balanced current transformer cannot sense the ground fault current. In order to sense the ground fault current with a core-balanced current transformer, it is necessary to pass both line and load conductors through the core-balanced current transformer to measure an unbalanced current which is the ground fault current.
In 3-phase four-wire multi-grounded distribution systems, it is difficult to distinguish ground fault currents from load currents. Depending on the location at which a ground fault occurs, the amplitude of the current may be comparable with a load current or even with a large short-circuit current. Therefore, if the detectable level is lowered to detect weak current such as an arcing ground fault current of about 1 ampere, then the device is tripped at normal load currents, thus presenting problems. If the detectable level is increased to prevent the device from being tripped at normal load currents, then no arcing ground fault currents can be detected.